When a phosphor or other luminescent material emits light, in general, it emits light according to Stokes' Law, which provides that the wavelength of the fluorescent or emitted light is always greater than the wavelength of the exciting radiation. While Stokes' Law holds for the majority of cases, it does not hold in certain instances. For example, in some cases, the wavelength is the same for both the absorbed and the emitted radiation. That is, the efficiency appears to be perfect or unity. This is known as resonance radiation.
In other cases, Stokes' Law does not hold where the energy emitted is greater than the energy absorbed. This is known as anti-Stokes emission. This may be due, at least in part, to the fact that at the time a photon is absorbed by a molecule, a collision with one or more other molecules adds extra energy to the absorbing molecule. Consequently, the absorbing molecule receives excess energy over and above what it received from absorbing the photon, and it is promoted to a higher excited state than it would have been promoted to from the original absorption event alone. The molecule is then free to decay from this super-excited state and emit a more energetic photon than it originally absorbed. Anti-Stokes materials typically absorb infrared radiation in the range of about 700 to about 1300 nm, and emit in the visible spectrum.
The use of anti-Stokes luminophores was first mentioned in 1974 by Malmberg et al., in Swedish patent application nos. 7705938-4 and 7413480-0, which correspond to German Patent No. 2,547,768. Further, Bratchley et al., British Patent Nos. 2,258,659 and 2,258,660, proposed oxysulfide anti-Stokes luminophores are materials that could be used to code security documents. However, Malmberg and Bratchley only suggest using Y2O2S as a basic lattice material.
Muller et al., WO 00/60527, proposed anti-Stokes luminophore oxysulfide compositions that are stimulated with a pulsed 980 nm laser in order to increase the reliability of detection. Muller also suggested that the excitation conditions should be such that Class I lasers (which have an output power less than 1 mW and are therefore, not harmful to the human eye) can be used. This can be achieved by accurately matching the pulse frequency and the pulse interval to the build-up characteristics of the luminophore used. The laser parameters are adjusted so that the resulting luminescence intensities are at least 50%-90% of the saturation intensity, i.e., the intensity at the steady state laser excitation, of the respective fluorescent substance. Muller suggests Y2O2S:Yb, Er, Y2O2S:Yb, Tm and Gd2O2S:Yb, Er are suitable luminophores.
There are innumerable different types of documents and things which are subject to counterfeiting or forgery, and many different techniques and devices have been developed for determining the authenticity of a document or a thing. By way of example only, documents which are particularly in need of authentication include bank notes, identification papers, passports, packagings, labels and stickers, driver's licenses, admission tickets and other tickets, tax stamps, pawn stamps, and stock certificates. As used herein, the term “secured document” includes any document or thing which is provided with a distinguishing device (whether printed or not) which can be used to authenticate, identify or classify the document.
Furthermore, in addition to determining the authenticity of a secured document, it is sometimes useful to also determine the nominal value of the document or the nature of the document. For example, in a postal system, it is not only necessary to establish the authenticity of the postal stamps and/or release stamps, it may also be beneficial to determine the value of the postage stamps as they are passed through a postal sorting machine.
Accordingly, as used herein, the term “authentication element” is intended to refer to any “device” which may be printed on, or otherwise attached to, a secured document for the purpose of authenticating the document or for the purpose of determining its value and/or type or any other characteristic. Likewise “authenticity” is meant to encompass value, type or other characteristic of a secured document, as well as the genuineness of a document or thing.
It is known to provide secured documents such as bank notes with an authentication element in the form of a distinctive luminescent ink which, when excited by a light of a predetermined wavelength, will emit a distinctive low intensity radiation that can be detected and analyzed as a means for authenticating a secured document. German Patent No. DE 411 7911 A1 discloses such a system which includes a conically expanding fiber optical waveguide and an optical processing system. The radiation from the object to be tested can be collected over a large spatial angle with the narrow cross-sectional end of the fiber optical waveguide. Because of the cross sectional transformation, the radiation emerges from the fiber at a significantly smaller angle, which is coordinated with the cone angle of the optical processing system.
With such a system it is possible to detect relatively low intensity distinguishing luminescent authenticity elements. However, the magnitude of the distinguishing luminescent elements must exceed a certain threshold. The system is therefore still relatively insensitive. Because of the use of a conical fiber, there is also the disadvantage that only a small region of the document can be monitored and checked. Moreover, the system may fail if the authenticity element is disposed at certain places in the document. Further, documents such as postage stamps cannot be identified with this arrangement at the high speeds customary in sorting, distributing and/or counting machines. In the case of laser excitation, characteristic pulse responses, which are of decisive importance for identifying authenticity, also may not be recognized and evaluated.